I highly doubt that it's a keylogger. There are a multitude of potential reasons why it's not keeping your credentials.

Here are some different things to try, start with one and work your way down:

1. http://support.microsoft.com/kb/290684 (*WARNING - Some of these fixes require registry edits. Make sure to follow the directions or you could seriously damage your computer.)
2. http://www.howtogeek.com/howto/windo...word-on-vista/
3. Delete the Outlook Profile, Restart PC, and create a new one. Sometimes that helps.
4. Try uninstalling and then reinstalling outlook. *Make sure you back up your PST file just in case (Here's how: http://support.microsoft.com/kb/287070).